1. Field of the Invention
The present invention relates generally to the field of flue gas desulfurization absorbers and, in particular, to a new and useful absorber arrangement for use in high velocity absorbers having straight, non-flared tank walls.
2. Description of the Related Art
Commercialization and development of high velocity absorbers is pursued because of the economic advantages they offer such as lower capital cost, less real estate requirements, shorter and more compact absorbers, and improved SO.sub.2 removal efficiency. On the other hand, high velocity has some disadvantages such as increased resistance to gas flow and increased sensitivity of the system to changes in the hydraulic behavior of the gas and liquid phases. Physical model studies show that the gas velocity through the inlet of the absorber affects gas distribution in the absorber and reflects on the performance and behavior of the absorption zone and mist eliminator.
Regardless of the physical shape of the absorber, the resistance to gas flow is categorized as either useful resistance or parasitic resistance. Useful resistance is converted directly and entirely into scrubbing efficiency and participates in gas redistribution such as the absorption zone pressure drop. Parasitic resistance is expended to conduct the gas through the absorber confines without effective participation in the chemical process. The inlet and outlet resistances are good examples of this type of resistance. The use of turning vanes or other gas distribution devices is a simple solution to reduce outlet resistance. However, the inlet pressure drop is not easy to reduce because it affects the gas and the scrubbing liquid interaction throughout the absorber.
Traditional absorber inlets vary in shape and size but the shape of the inlets is basically the same. FIG. 1 shows the commonly offered inlet design (without protective awning). In this design, the liquid flowing off the absorber walls 12 or sprayed by nearby spray headers, falls into the inlet 14 and forms a solid growth known as "elephant ears". To overcome this problem and as shown in FIG. 2, protective intrusive awnings 16 were placed on top of the inlet 14 (see U.S. Pat. No. 5,281,402 to Gohara et al.). The awning diverted the contact point between the hot gas and the liquid curtain flow into the center of the absorber. Elephant ears formation is averted because gas humidification occurs in an area where there is minimum contact between the hot gas and the solid surfaces. This design has been proven functional at the traditional gas velocities and when the spray zone resistance is large enough to affect even distribution before the gas reached the mist eliminator further up in wall 12. As the gas velocity increases, however, the curtain resistance adds significantly to the overall system parasitic pressure drop and distortions to gas flow pattern becomes more critical.
While the liquid curtin is needed to humidify and help gas redistribution, it has two significant drawbacks. It significantly increases the inlet pressure drop compared to a no awning inlet, and it distorts the flow pattern as the gas rises through the absorber.
In a new generation of high velocity absorbers, gas velocity is set between 15 and 20 feet per second. Minor distortion in the gas flow pattern results in localized high gas velocities approaching or exceeding the critical velocity of the mist eliminator and may result in functional failure of the mist elimination device.
To overcome the negative effects of high gas velocity in the inlet, one could increase the inlet's flow area and limit the gas velocity to the conventional 3,000 feet per minute. This solution, while simple and practical, will result in a larger inlet aspect ratio and increases the absorber height. An increase in absorber height minimizes the advantages gained by high velocity scrubbing. Other options include advanced low pressure drop gas inlets for the new generation of high velocity absorbers, or the use of available means within the system to redistribute the gas flow without significant increase in inlet resistance.
A current inlet design of The Babcock & Wilcox Company requires the installation of the protective awning 16 on top of the inlet 14 to deflect the slurry away from the hot flue gas flow and prevent the deposition of solids at the wet/dry interface. However, at high absorber gas velocity, obstruction of the gas path by the high density liquid curtain deflects the gas to the sides causing a momentary increase in gas velocity, an increase in pressure drop, and possible distortion.
Recent model studies and operating experience teaches that between 1 and 12.5 feet per second, the current inlet designs provide good gas distribution across the absorber at or below 3,000 feet per minute inlet velocity. The good gas distribution is provided partially by the resistance of the liquid curtain, falling off the awning to the entering gas. The primary function of the awning is to provide protection against inlet wetness and to provide ample resistance to slow down the entering gas, thus allowing the gas adequate time to redistribute itself across the absorber flow area. At a gas velocity below 12.5 feet per second reasonable gas distortion in the absorber will not approach the critical failure velocity of the mist eliminator.
As the gas velocity increases above 12.5 and approaches 20 feet per second or more, the resistance of the liquid curtain falling off the awning becomes significant and magnifies the effects of gas flow distortion.
Several attempts were made to reduce the resistance of the awning first by introducing a new generation of non intrusive awnings. In these designs, the awning is removed from the inlet's gas stream and placed above the inlet. See for example, U.S. Pat. No. 5,403,523 to Strock, et al.; and U.S. Pat. No. 5,558,818 and 5,648,022 to Gohara et al. Each of these developments contributed to the reduction of the inlet's parasitic pressure drop caused by the intrusion of the original awning into the gas flow path. These designs, however, added 3 feet to the height of the absorber and none of them totally eliminated the effect of the heavy density liquid curtain.
These prior efforts were steps in the right direction to reduce the inlet's parasitic resistance; however, in every case the curtain resistance remained the same. Considering that one inch (water) of pressure drop is evaluated at $1 million over the life of the plant, reduction of the parasitic resistance of the absorber provides a significant competitive edge. Table 1 compares the pressure drop of an inlet with and without awning.
TABLE 1 Comparison of Inlet Pressure Drop for No Awning and Awning Designs* Non-Intrusive Description Intrusive Awning Awning No Awning Inlet Pressure 4.59 3.50 2.50 Drop (Inch Water) *Inlet velocity 3,600 feet per minute, liquid flux 60 gpm per square foot, absorber velocity 15 feet per second.
It should also be noted that the prior solutions for absorbers are adapted only for absorbers with flared walls, and do not apply to absorbers with straight, vertical walls.